\documentclass{article}\begin{document}
\begin{theorem}

\declare{syntax={postfix,108,"+"}, meaning=arith1.unary_plus}
\declare{syntax={infix,100,"/",l}, meaning=artih1.divide}
\declare{syntax={infix,108,"+",l}, meaning=arith1.plus}
\declare{syntax={infix,100,"*",l}, meaning=arith1.times,thisis=nonsemanticdata}
$(2++3)$
\end{theorem}
\end{document}

